An Integrated WAG Characterization Study for an Offshore Oilfield

Abstract

Water-Alternating-Gas (WAG) injection is an enhanced recovery method that is being applied in some brazilian offshore oilfields as an alternative to combine effective pressure maintenance policies, flexible produced gas management strategies and increased recoveries. In this technology, gas plays the role of reducing residual oil saturation while water controls, by multiphase flow-in-porous-medium effects, gas adverse mobility. In addition to the extra engineering tasks needed to design development plans and production facilities, an intense characterization programme should be put in practice in order to reduce risks, increase predictability and optimize WAG floods. Much more complex laboratory tests than the ones usually performed for continuous water/gas floods should be done, particularly to tackle multiphase flow in porous medium phenomena. The proper characterization, modeling and simulation of these effects are vital for a representative WAG design and evaluation. This paper presents the results of a pragmatic and integrated laboratory characterization, modeling and simulation study for an offshore oilfield that was focused on relative permeability hysteresis effects. Firstly, it summarizes the best relative permeability hysteresis model (Larsen & Skauge) available in most commercial reservoir simulators. Then, it describes a special laboratory WAG multiphase flow-in-porous-media characterization program, designed and executed to obtain the parameters of this hysteresis model. Experiments were done at reservoir conditions and with rock and fluids from an actual offshore field. Following, results of these tests were evaluated in light of Larsen & Skauge model, in order to confirm its validity. Finally, WAG simulations of a sector of the target field were performed to access the impacts of including or neglecting relative permeability hysteresis phenomena. Conclusions of this work contribute to increase predictability and reduce uncertainties related to field-scale implementation of WAG technology. Knowledge acquired from this study promoted a better estimation of oil recovery, gas production and overriding. It also supports the design of subsea and topside equipment, which is critical in the offshore scenario.

State of the Art on Relative Permeability Hysteresis in Porous Media: Petroleum Engineering Application

“…(2) Decrease in recovery [11] Several numerical simulation studies have found that models considering hysteresis in relative permeability provide predictions of recovery rates closer to actual conditions, with hysteresis in simulations leading to a decrease in recovery rates. Reasonably modeling hysteresis phenomena in the alternating gas-water process can significantly enhance the stability and accuracy of production outcomes.…”

“…(3) Phase trapping [11] In many reservoir applications, phase trapping has been demonstrated in the literature as a significant factor for reducing productivity. The hysteresis effect of relative permeability has been found to cause trapping of the wetting phase by the non-wetting phase.…”

“…(4) Early breakthrough of gas and water [11] Numerous experts have studied the impact of hysteresis on breakthrough times, with research indicating that considering hysteresis effects results in earlier breakthrough times for gas and water compared to conventional models that do not consider hysteresis effects. Failing to consider hysteresis effects may lead to erroneous estimates of the time for water breakthrough, preventing timely water shutoff measures and thereby hindering the maximization of economic benefits.…”

“…At the front edge of the CO 2 slug, gas continues to displace the wetting phase during drainage, while at the rear edge, water displaces gas during imbibition, leading to gas phase fragmentation and subsequent trapping. As the slug migrates upwards, some CO 2 is trapped as residual immobile phase in pore spaces, effectively isolating CO 2 in the rock until the immobile gas dissolves over geological time [11,73,74,76,77,[85][86][87].…”

“…Ignorance of the relative permeability hysteresis effect of gas water alternation will misinterpret the production/experimental result [10]. Not considering hysteresis of relative permeability will underestimate the recovery factor and storage capacity in WAG-related CCUS application [11].…”